The present invention is directed generally to optical transmission systems. More particularly, the invention relates to amplifying optical signals in optical transmission systems and controlling nonlinear interactions between signal channels, as well as the amplifier spacing in the optical systems.
Optical communication systems transmit information by generating and sending optical signals corresponding to the information through optical transmission media. Information transported by the optical systems can include audio, video, data, or any other information format. The optical systems can be used in long distance and local telephone, cable television, LAN, WAN, and MAN systems, as well as other communication systems.
Information can be transmitted optically using a broad range of frequencies/wavelengths at high data rates and relatively low cost, which are desirable attributes for high capacity, transmission systems. Also, multiple optical wavelengths that are combined using wavelength division multiplexing (“WDM”) techniques into one optical signal that can be transmitted through one optical fiber, which further increases the data carrying capacity of optical systems. As such, optical fiber transmission systems have emerged as a cost-effective alternative to electrical systems for providing high capacity, communication systems.
However, optical transmission systems are not free from attenuation and various forms of degradation that limit the performance of the systems. For example, optical fiber is not a perfect transmitter of electromagnetic radiation in the optical spectrum. Thus, the intensity of an optical signal will attenuate as it travels through the fiber, due to scattering from fiber material imperfections and other degradation mechanisms. In addition, optical noise, from signal attenuation, chromatic dispersion, nonlinear interactions, and other sources, will accumulate and propagate in the fiber along with the signal further degrading the quality of the signal. Furthermore, optical systems generally are not operated in the identical manner, which requires that interfaces be provided to interconnect different optical systems.
It is therefore necessary to regenerate optical signals being transmitted through the optical system to overcome the three primary limitations on optical transport, namely: 1) optical signal attenuation, 2) optical noise accumulation, and 3) optical system interconnectivity. The regeneration of optical signals can be performed either optically or electrically.
The development of optical amplifiers greatly reduced the cost of optical systems, and WDM systems in particular, by essentially eliminating the need to electrically regenerate signal merely to overcome signal attenuation. While the development of optical amplifiers has greatly reduced the equipment costs associated with amplifiers in optical systems, there remain substantial operational costs. Real estate and building acquisition and maintenance costs associated with optical amplifiers can be a sizable portion of the optical system operational costs, which suggests maximizing the distance between optical amplifiers in an optical system. However, maximizing the distance between amplifiers can reduce the maximum distance that optical signals can be transmitted before having to be regenerated to overcome accumulated optical noise.
Additional cost savings in the system can be achieved by increasing the transmission capacity of existing optical fiber compared to installing additional fiber in the system. Therefore, it is desirable to increase the information bit transmission rate in the fiber and the number of wavelengths used to carry information to increase the information carrying capacity of the fiber. However, increasing the bit rate of each channel generally requires the channel spacing to be increased. Furthermore, increasing the bit rate and/or the number of channels often requires that the transmission distance to be decreased, which increases the system cost.
The ability to increase the capacity of the system is limited by the optical signal degradations that occur in the system. Optical signal degradation occurs via numerous mechanisms during transmission in optical fiber. The primary mechanisms are chromatic dispersion and various nonlinear interactions, such as four wave mixing. The degradation caused by these mechanisms increases proportionally as either the bit transmission rate or the number of the channels within a wavelength range is increased.
Generally, information carrying signal wavelengths, or signal channels, are launched into the optical fiber at a maximum signal channel power. The signal channel power decreases as it travels through the fiber until it reaches a minimum signal power, at which time it must be amplified to prevent degradation of the signal. Thus, for a given channel spacing, the maximum signal launch power, minimum signal power, and the attenuation of the fiber establish the maximum amplifier spacing in the system.
The maximum signal launch power is limited to powers below which nonlinear interactions do not cause unacceptable signal degradation. The spacing of the channels as well as other factors, such as the signal channel polarization, affect the maximum signal launch power. Safety concerns may further limit the total power that can be launched into the fiber. The minimum signal power is determined based on the minimum acceptable signal to noise ratio required to reliably transmit information through the system.
The development of optical fiber technology has resulted in fiber having very low attenuation levels (0.25 dB/km) compared to older fiber designs (>0.30 dB/km) in the wavelength range around 1550 nm. The lower loss fiber allows amplifiers to be separated by greater distances for signal transmitted at a given power level and/or lower power signals to be transmitted over greater amplifier spacings.
The different optical fibers used in systems introduce different amounts of chromatic dispersion as a function of optical wavelength. Chromatic dispersion in standard single mode, silica-based, optical transmission fiber, such as SMF-28, generally varies as a function of wavelength. Average dispersion values in standard silica-based fiber are approximately −17 ps/km/nm in the 1550 nm low loss optical transmission window, whereas the wavelength at which zero dispersion occurs (the “zero dispersion wavelength”) is typically around 1300 nm.
In optical transmission systems employing standard transmission fiber, chromatic dispersion can severely degrade the optical signal quality and thereby limit the maximum transmission distance. Numerous methods have been developed to effectively counteract dispersion in the standard fiber. For example, dispersion compensating (“DC”) fibers have been developed that have high dispersion rates, on the order of 102 pm/km/nm, that are opposite in sign from transmission fiber. The DC fibers can be inserted into the transmission fiber at various locations to maintain the absolute dispersion in the system to within a desired range.
New transmission fibers have been designed to minimize the chromatic dispersion in the 1550 nm window. The new fiber types, generally referred to as low dispersion (“LD”) fiber, have much lower dispersion than standard fiber in the range ±5 pm/km/nm for non-zero dispersion shifted (“NZ-DS”) fiber, such as Truewave, LEAF, and LS, and even lower for zero dispersion shifted (“DS”) fibers. The LD fibers facilitate the optical signal transmission over substantially longer distances before substantial signal degradation occurs as a result of chromatic dispersion. In addition, DC fibers also have been developed to compensate for dispersion in LD fibers.
However, a problem with LD fiber arises from the interrelation of chromatic dispersion and nonlinear interactions. High rates of dispersion tend to decrease nonlinear interactions between closely spaced wavelengths, because the relative velocity of the channels, or “walk-off”, decreases the interaction time between the channels. In LD fiber there is much less walk-off between adjacent closely spaced channels resulting in longer interaction time between channels, which increases the nonlinear interaction and resulting degradation of the signal channels.
Nonlinear interactions in LD fiber can be decreased in the system by increasing the spacing between adjacent channels or decreasing the signal channel power. However, increasing the channel spacing generally reduces the total number of channels in the system. Likewise, decreasing the signal channel power generally requires a corresponding decrease in the amplifier spacing and the transmission distance between electrical regeneration sites.
The limitations imposed by nonlinear interactions become increasing problematic for LD fibers at relatively low transmission capacities and short distances. The current methods of reducing the nonlinear interactions in LD fiber greatly decreases the optical system capacity and performance compared to equivalent systems employing standard fiber.
The capacity limitation resulting from non-linear interactions is not limited to LD fiber, but also plagues standard fiber, transmission systems. This limitation will require increased capital and operating expenditures to expand optical communication system capacity. Accordingly, there is an extremely pressing need to overcome the fiber limitations in current optical systems to allow continued growth in communication systems and communications based technology.